Microwave heating of substances under hydrostatic pressure



Aug. 20, 1968 M. R. JEPPSON ETAL 3,393,251

MICROWAVE HEATING 0F SUBSTANCES UNDER HYDROSTATIC PRESSURE OriginalFiled May 4, 1964 2 h tsh 1 FIE-l 8/ WA TE R PRODUCT IN fiODl/CT OUTSUPPLY HIGH PRESSURE GAS SUPPLY u /56 52 L 54 MICROWAVE 72 u 2 URCE H lK E g 43 n l 72 p r f 37 TEMPERATURE 56 TEMPERATURE TEMPERATURE 7 E ACONTROL I CONTROL 111 CONTROL m EMP R run 27 29 CONTROL 1r 3/ "7 @w, i Qi 4/ 48 5 INVENTOR.

MORRIS R. JEPPSON JOHN C. HARPER 0, 1968 M. R. JEPPSON ETAL 3,398,251

MICROWAVE HEATING OF SUBSTANCES UNDER HYDROSTATIC PRESSURE OriginalFiled May 4, 1964 2 Sheets-Sheet 2 35" :I: l3= 4. I

TEMPERATURE x m REGULATOR I 8 //4 1/3 (H6 96 man PREssuR MICROWAVE 8SUPPLY I SOURCE 2 TEMPE RA TURE RE GULA TOR 11' 'iiiiiiiiiiiiiiiib '6]'0] 'G-I'GI'GYEEEE S =5 :2 as t :3" I 66 59 Q =2: E sl 5 5 E 88 Q E pg E/-7/ Q INVENTOR. -I MORRIS R. JEPPSON l i uomv c. HARPER F;

United States Patent 3,398,251 MICROWAVE HEATING 0F SUBSTANCES UNDERHYDROSTATIC PRESSURE Morris R. Jeppson, Alamo, and John C. Harper,Davis, Califl, assignors to Cryodry Corporation, San Ramon, Calif., acorporation of California Original application May 4, 1964, Ser. No.364,405, now Patent No. 3,335,253, dated Aug. 8, 1967. Divided and thisapplication Mar. 15, 1967, Ser. No. 643,757

6 Claims. (Cl. 21910.41)

ABSTRACT OF THE DISCLOSURE Microwave heating of substances underpressure by passing said substances through a liquid to a water-freeregion therein and at a depth to create a hydrostatic pressure on saidsub-stances, and then microwave heating said substances in said region.

This application is a division of application Ser. No. 364,405, filedMay 4, 1964, now US. Patent 3,335,253.

This invention relates to the treatment of substances in a high pressureenvironment and more particularly to a method and apparatus utilizingmicrowave energy and high pressure conditions to heat food products andthe like to high temperatures without adverse effects on the products orproduct containers.

In the commercial processing of foods it is frequently desirable to heata product rapidly and uniformly to temperatures above the atmosphericboiling point of water or other constituents thereof. A brief hightemperature heat treatment. is desirable, for example, in sterilizingmany foods prior to, or after, canning. Similarly very high temperaturesmay -be preferable for blanching or cooking certain foods. Hightemperature food processing of this type generally requires a highpressure environment to prevent boiling, undesired cooking, excessmoisture loss, container rupture where the product is heated aftercanning or packaging, or other adverse effects.

Similar requirements may be present in processing products other thanfoods. To produce styrafoam, for example, a small amount ofdetergent-water solution is mixed with the styrene particles and themixture must be heated to about 250 F. without evaporating the water.Accordingly it is necessary that the heating be conducted in apressurized environment.

One technique for providing such an environment is that of heating theproduct after it has been sealed in a rigid high strength containerwhereby the temperature rise of the natural moisture content of theproduct acts to build up the internal pressure within the container. Anexample of such a process is disclosed in copending application SerialNo. 274,648, filed April 22, 1963, by the present co-inventor, Morris R.Jeppson, and entitled Process for Sterilizing Food Products. In theprocess disclosed therein, food products are sterilized by a shortexposure to microwaves after being canned in conventional permanentcontainers, such as glass jars, of a type capable of withstanding abrief internal pressure rise.

The techniques of the above-identified copending application cannot beused if the product is to be heated in an unpackaged condition or if itis packaged in low strength or flexible containers such as plasticpackets, tubes, paper cartons or the like. In addition, the techniquebecomes relatively expensive for particular processes which require icean unusual degree of heating as specialized nonstandard containers maybe required.

An alternative technique, which is not limited by nature, or absence, ofthe product container, is that of conducting the heating within a highpressure chamber. In many instances it is highly desirable that the heattreatment be performed on a continuous process basis and thus some formof pressure lock must be provided at opposite ends of the chamber.Various forms of mechanical lock, through which products may becontinuously passed into and out of a pressure chamber, are known to theart however such structures tend to be complex and subject toconsiderable maintenance effort.

The need for mechanical pressure locks may be avoided by submerging theproduct under a volume of water during heating thus making use ofhydrostatic pressure. Hydrostatic cookers of this type are known to theart and consist essentially of a conveyer for carrying the products intoand out of an upright water filled tunnel. Heating in these systems isaccomplished by passing the product through a hot water or steam regionat the lower portion of the tunnel.

As discussed in the above identified copending application, the brief,intense and uniform heating needed for sterilizing certain productswithout deleterious effects thereto cannot be accomplished by applyingheat solely to the surface of the product such as occurs in hot water orsteam heating. In order to sterilize the interior of the product, thesurface regions thereof must be overheated from the standpoint ofoptimum quality. In the method of the above identified copendingapplication, the desired high temperature short time heating of theproduct is effected by microwave energy which provides a uniform heatingthereof. However, as previously discussed, the technique requires apressure environment and is limited to products which have been packagedin high strength containers.

The present invention provides a technique for uniformly heating foodproducts and the like to high temperatures which does not require highstrength containers but is applicable to products which are unpackagedor contained in any of various types of relatively weak container. Inparticular, the invention provides for microwave heating, which may beaccompanied by surface heating, under hydrostatically derived pressure.

This result cannot be achieved by simply combining the separatetechniques of microwave heating and hydrostatic eooking as presentlypracticed. Microwave energy preferentially heat-s water and as mostfoods contain a substantial moisture content it is this property thatmakes microwave heating particularly adaptable to food processing.However this property seemingly rules out the practical use of microwaveheating in a hydrostatic apparatus. If microwave energy were to beinjected into a water filled tunnel, a major portion of the energy wouldbe absorbed by the water with the result that the process would notgenerally be feasible from the economic standpoint.

Accordingly the present invention provides for the exclusion of waterfrom a lower region of a hydrostatic tunnel while retaining the highpressure environment provided by a volume of liquid above the region andfurther provides for microwave heating of products in passage throughthe water free region.

Water is excluded from at least the microwave heating section of thetunnel by substituting a fluid therefor which is non-lossy relative tomicrowave energy. Mineral oil, for

example, is relatively transparent to microwave energy and issufiiciently heavy to produce a substantial hydrostatic pressure head.

Thus in a typical embodiment of the invention, a U- shaped tunnel isprovided with a conveyor arranged to carry products therethrough. Amicrowave source is coupled to the lower section of the tunnel in such amanner as to inject energy into the tunnel in a direction substantiallynormal to the axis thereof. The walls of the microwave heating sectionof the tunnel are made of electrically conductive material so that theenergy is repeatedly reflected back and forth across the axis of thetunnel and repeatedly passes through the products being conveyedtherethrough.

To provide hydrostatic pressure, both upwardly directed legs of thetunnel are filled with liquid and the microwave heating section isfilled with a fluid which may not necessarily be the same. Thus in onearrangement the entire tunnel including the heating section is filledwith mineral oil. Alternately, one or both of the vertical legs of thetunnel may be filled with water while the heating section contains oil,interchange of the oil with the denser overlying water being preventedby providing a small gasfilled inverted U-shaped loop in the tunnel atthe transition point. In still another arrangement, the microwaveheating section is in a gas-filled inverted U-shaped loop at the bottomof the tunnel with both upwardly directed legs thereof being filled withwater.

More economical processing is obtained if the microwave heating issupplemented by surface applied heat so that the function of themicrowave becomes largely that of providing for a uniform temperaturerise throughout the product. The invention readily lends itself to suchsupplemental heating inasmuch as the liquid in the entrance leg of thetunnel may be at a controlled high temperature. Moreover, where agradual heating of the product is desired, an appropriate temperaturegradient may be established in the entrance leg of the tunnel.

Preheating in the above described manner brings the temperature of thesurface of the product to the desired level as it enters the microwaveheating section. The exposure time in the microwave section may then bearranged to bring the temperature of the interior of the product up tothe desired level. Usually a holding period at the high temperaturelevel is required and this also may readily be provided by maintainingthe fluid at the high temperature in the section of the tunnel whichfollows the microwave section.

Product cooling may be accomplished in an essentially similar manner bymaintaining a temperature gradient in the liquid column of the exit legof the tunnel. As cooling may require more time than heating, the exitleg of the tunnel may be less steeply inclined than the entrance leg toprovide for a relatively longer travel time therethrough. Forced coolingby such means is frequently necessary where the product is in acontainer as the heat induced pressure therein must be reduced as theexternal pressure drops to avoid rupture of the container.

Thus the invention provides a very convenient and practical techniquefor such processes as the high temperature short time sterilization offoods wherein cooking is minimized and which is not dependent on the useof high strength containers. Meats, vegetables and fruits may besterilized after packaging in flexible plastic pouches, for example,with a briefer but more intense heat treatment than has heretofore beenfeasible. Such treatment generally produces a higher quality productfrom the standpoint of taste, appearance and nutritional content.Additional advantages result in the case of certain specific products.The high temperature treatment has a tenderizing effect on some meats.In the case of fruits and meats, juice loss is minimized owing to thereduced pressure gradient within the product.

It will be evident that the invention is also applicable to the heattreatment of products which are not foods but which are similar in thatmoisture loss, container failure or other undesirable effect would occurif the heating were conducted at atmospheric pressure.

Accordingly, it is an object of this invention to provide a method andapparatus for heating substances with microwave energy under highpressure conditions.

It is an object of the invention to provide for eflicient microwaveheating within a hydrostatic pressure chamber.

It is an object of the invention to provide a method and means forrapidly and uniformly heating products above the atmospheric boilingpoint of constituents thereof without adverse effects.

It is :an object of this invention to provide means for heatingsubstances which are packaged in weak or flexible containers to hightemperatures without damage thereto.

It is an object of the invention to provide a method and apparatus forrapidly and uniformly heating food products and the like to temperaturesabove the atmospheric boiling point of water without requiring highstrength containers, mechanical pressure locks, or the like formaintaining the necessary high pressure environment.

It is another object of the invention to provide a method and apparatusfor the high temperature short time sterilization of foods which is notrestricted by strength or rigidity characteristics of the food containeror package.

The invention, including further objects and advantages thereof, will bebetter understood by reference to the following specification and theaccompanying drawings of which:

FIGURE 1 is a partially schematic view of a first embodiment of a foodprocessing installation for practicing the invention,

FIGURE 2 is a cross section view taken along line 22 of FIGURE 1 andshowing details of the microwave heating chamber thereof,

FIGURE 3 is a partial section view taken along line 33 of FIGURE 2, and

FIGURE 4 is a partially schematic view of a second form of foodprocessing installation embodying the invention.

Referring now to the drawing, and more particularly to FIGURE 1 thereof,there is shown a continuous process tunnel 11 through which products maybe passed on a continuous basis. Tunnel 11 includes a vertical entrancesection 12 which continues, at the lower end thereof, into a horizontalmicrowave heating and holding section 13. A lower product exit section14 of the tunnel slants upwardly from the end of section 13 oppositesection 12 and connects with an upper exit section 16 through arelatively short downwardly slanted transition section 17. Forconvenience in handling the product, the upper ends of both the entrancesection 12 and exit section 16 may be transpierced through an elevatedplatform or walkway 18.

The essentially U-shaped configuration of the tunnel 11 allows a liquidfilling to be retained therein for developing the desired high pressurewithin the microwave heating section 13. To avoid excessive microwavepower loss, at least that portion of the liquid which fills the heatingsection 13 must be a substance which is not lossy to electromagneticenergy in the frequency range from about 200 megacycles to 10,000megacycles, the use of water in this section of the tunnel being ruledout by this requirement. Suitable liquids for this purpose include, butare not limited to, mineral oil, transformer oils or vegetable oils.

In the present embodiment. the entrance section 12, heating and holdingsection 13, and the lower product exit section 14 of the tunnel 11contain a filling 19 of mineral oil. To avoid the use of anunnecessarily large quantity of relatively expensive oil, the upperproduct exit section 16 is filled with water 21. As the section 16 isremote from the microwave heating section 13, the water filling 21 doesnot significantly affect the microwave heating.

To prevent the interchange of oil 19 with the over-lying volume of water21, a liquid free region 22 is maintained at the juncture of lower exitsection 14 with transition section 17. To maintain the region 22, gasfrom a high pressure supply 23 is admitted to the region through a valve24. By thus raising the pressure in region 22, the upper level of theliquids in sections 14 and 17 is forced below that of the interveningtunnel wall portion 26 and no liquid interchange can occur. It will beapparent that a similar liquid free region may be established at theentrance portion of the tunnel, by providing a downwardly directedtransition section therein, if it should be desired to use water at bothends of the tunnel.

As microwave heating is inherently more expensive than most othermethods, it is desirable that conventional heating be employed to heatthe surface regions of the product, the function of the microwave beingto rapidly and uniformly heat the interior of the product. In addition,it is desirable that the liquid within the microwave heating region beat approximately the required product temperature to prevent the loss ofmicrowave generated heat therefrom. Accordingly means are provided formaintaining the oil 19 in the microwave heating section 13 of the tunnel11 at the required temperature. In addition, means are provided formaintaining a temperature gradient in the entrance section 12 whichincreases in the downward direction to gradually bring the product up tothe temperature of the microwave section 13 and for maintaining asimilar temperature gradient in the tunnel exit sections 14 and 1-6 tocool the product as it moves upwardly and experiences a progressivelydiminishing pressure. In the absence of such cooling, product containersmight rupture upon being removed from the high pressure region.

To control the temperature of the oil within the heating and holdingsection 13 of the tunnel 11, a variable flow rate pump 27 withdraws aportion of the oil, through an intake 28 at the output end of section 13and through a filter 29. The flow from pump 27 passes through a suitabletemperature regulating means 31 and is returned to the opposite end ofsection 13 through an output tabulation 32. Upon returning to tunnelsection 13, the circulated oil intermixes with the remainder and thus anappropriate adjustment of the regulator 31 results in control of thetemperature of the entire oil content of section 13. The regulator 31may, in some instances, be of a type capable of cooling, as well asheating, the oil as the heat input to the tunnel section 13 frommicrowave energy may be greater than the heat loss therefrom.

To maintain the temperature gradient in tunnel entrance section 12, asecond pump 33 withdraws oil from the top of section 12 through anintake 34 and filter 36. The flow from pump 33 passes through a secondtemperature regulator 37 and is returned to the bottom of tunnel section12 through an outlet 38 thereat. The oil from regulator 37 is fed intothe lower end of tunnel section 12 at a temperature approximatelyequivalent to that of the oil within microwave heating section 13.However, as the oil moves upwardly in tunnel section 12, heat isdelivered to the incoming cool product so that the oil temperatureprogressively decreases and may be close to that of the ambienttemperature at the top of the tunnel.

A reverse process occurs at the product output end of the tunnel 11,i.e. heat is transferred from the product to the liquid. Economicaloperation may be achieved by transferring the heat content of the outputtunnel liquid to the entrance section 12 liquid. This is accomplished inthis embodiment by passing the oil flow between pump 33 and secondtemperature regulator 37 through first and second heat exchangers 39 and41 respectively. As Wiil hereinafter be discussed in more detail, theheated liquid-s from the output end of the tunnel 11 are also passedthrough the heat exchangers 39 and 41 to effect the desired heattransfer.

The temperature gradient in the lower tunnel exit section 14 ismaintained by a third pump 42 which withdraws oil through an intake 43at the lower end of section 14 through a filter 44. The flow from pump42 is partially cooled by passage through heat exchanger 39 and thenreturned to the upper end of tunnel section 14 through a thirdtemperature regulator 46 and outlet 47. The oil which is fed into theupper end of tunnel section 14 is initially at an intermediatetemperature and is subsequently further heated by the product so that ithas a temperature approaching that of the oil within microwave heatingsection 13 at the lower end of the section 14. Thus the function ofregulator 46 will generally be that of cooling the oil flow althoughthis is partially accomplished Within the heat exchanger 39.

An essentially similar system is employed to maintain the temperaturegradient in the upper product exit section 16. Thus a third pump 48withdraws cooling water 21 from an intake 49 at the upper end oftransition section 17 through a filter 51. The outflow from pump 48 ispartially cooled by passage through the second heat exchanger 41 and isreturned to an outlet 52 at the upper end of tunnel exit section 16through a fourth temperature regulator unit 53.

Under the usual operating conditions, the water temperature at intake 49is approximately that of the oil at the upper end of lower exit tunnelsection 14 so that there is an approximately continuous temperaturegradient throughout both exit sections 14 and 16. The water tem peratureat the top of upper exit section 16, determined by adjustment ofregulator 53, does not generally have to be as low as the ambientatmospheric temperature. In most instances, if the emerging product hasbeen cooled below the boiling point of its constituents, no deleteriouseffects result.

More precise temperature regulation may be obtained in the abovedescribed system if means are provided to reduce the interchange of oilbetween the several sections of tunnel 11. For example transversebaffles 54 may be disposed in the tunnel 11 at the lower ends ofsections 12 and 14, each bafile having an opening 56 just sufiicient topass the particular product undergoing processing.

Referring now to FIGURES 2 and 3 in conjunction with FIGURE 1, thetunnel section 13 includes an initial portion 57 into which microwave isinjected to heat the interior of the product and a second holdingportion 58 in which the product remains at the desired high temperatureinasmuch as heat loss is prevented by the surrounding hot oil 19. Toprevent the escape of microwave energy, the inner wall 59 of tunnelsection 13, and preferably the remainder of the tunnel 11 as well, isformed of electrical conductor such as stainless steel, copper oraluminum. To minimize the direct loss of heat a layer 61 of thermalinsulation is provided around the outside surface of the tunnel 11.

Microwave energy from a suitable source 62 is injected into portion 57of the tunnel section 13 through a slotted waveguide 63 which extendsalong the top of the tunnel portion 57 and which fits into a matchingopening 64 in inner wall 59 so that the waveguide effectively forms thecentral portion of the top wall of the tunnel. The side of the waveguide63 which faces the interior of the tunnel 11 is formed with a series ofspaced apart transverse slots 65 each of which radiates an increment ofthe microwave energy downwardly into the tunnel. The energy isrepeatedly reflected back and forth between opposite walls of the tunnel11 and thus repeatedly passes through the produce therein. With eachsuch passage through the product a portion of the energy is absorbed andconverted to heat. The structure and operation of the slotted waveguide63 microwave input for a heating chamber is described in more detail,and claimed, in copending application Ser. No. 308,284, filed Sept. 11,1963, by Morris R. Jeppson and Franklin J. Smith and entitled, PowerDistribution System for Microwave Process Chambers.

In this embodiment, the oil 19 is prevented from entering the waveguide63 by closure elements 66 disposed in the slots 65, the elements 66being formed of dielectric material and therefore being transparent tomicrowave energy. The waveguide 63 may however be filled with oil andstill function in a satisfactory manner.

Product may be carried through the tunnel 11 by any of a variety ofconveyor systems, the type selected being dependent on the configurationand characteristics of the particular product to be treated. A uniquesystem is utilized in this embodiment to sterilize foods such as slicedturkey, stews, beef Stroganotf, veal scallopini, etc. 67 after packagingin flexible plastic film packets 68. In particular the packets 68 areformed in a long strip by heat sealing spaced apart quantities of food67, in an unsterilized condition, between two ribbons of nylon film 69.For added strength, a welt 71 may be formed along the edges of the stripof packets 68 by folding over the edges of the film 59, however thestrain on the strip in being pulled through the tunnel 11 is less thanwould normally be the case owing to the buoyancy of the strip and itscontents in the liquid which fills the tunnel.

The strip of food packets 68 is continuously fed into entrance section12 of the tunnel 11 and removed from the upper exit section 16. A seriesof rotatable drums 72 are mounted within the tunnel to guide the strip,such drums being at the juncture of sections 12 and 13, the juncture ofsections 13 and 14, the juncture of sections 14 and 17, and at the baseof upper exit section 16. To support the strip of packets 68 above theentrance section 12, the strip is gripped between two adjacent paralleldrums 73 and a similar set of drums 73' engage the strip above the exitsection 16. Drive may be applied to drums 73', as indicated by arrow 74,to pull the product through the tunnel 11.

As shown in FIGURE 2 in particular, the drums 72 are journ'alled withinthe tunnel 11 by suitable bearings 76 and have end sections 77 ofgreater diameter than the central portion to contact the welts 71 of thestrip of packets 68. Flanges 78 are provided on the drum 72 to hold thestrip thereon.

Referring now again to FIGURE 1, a multiple nozzle 79, connected to awater supply 81, may be positioned to spray the strip of food packets 68as it emerges from the drive drums 73'. The spray provides final coolingand removes any remaining oil from the packets 68.

In operation, the food 67 is packaged as described above and theresultant strip of packets 68 is continuously pulled through the tunnel11 at a controlled rate by the drive drums 73. Pumps 27, 33, 42 and 48and temperature regulators 31, 37, 46 and 53 are operated to provide thedescribed temperature gradients in the tunnel entrance section 12 andexit sections 14 and 16 and to provide the uniform high temperature insection 13. Microwave source 62 is energized to inject energy into thefirst half of section 13.

Upon moving downwardly through tunnel entrance section 12, the surfaceof product is gradually heated while the product is subjected to aprogressively increasing pressure owing to the hydrostatic head. Thus,assuming that appropriate settings of the temperature regulators 'havebeen made, the product is heated above 212 F. without boiling or otherdisadvantageous results. On moving through the initial portion 57 of theheating section 13, the interior of the product is rapidly heated bymicrowave from source 62 to a temperature corresponding to that of thesurface of the product. The product then remains at the uniform hightemperature for the period required to travel through the holdingportion 58 of section 13.

The product then travels upwardly through exit sections 14 and 16 and iscooled by the progressively diminishing temperature of the oil 19 andwater 21 therein. Upon emerging from upper exit section 21, the productis washed by sprays 79. The individual food packets 68 may then be cutapart and the welts 71 trimmed therefrom.

It will be apparent from the foregoing description that a variety ofprocess temperatures and heating times may be arranged for byappropriate adjustments of the pumps, the temperature regulators and thespeed of the drive drums 73'. Specific temperatures and holding timessuitable for the high temperature short time sterilization of particularfood products are described in the hereinbefore identified copendingapplication Serial No. 274,648.

Referring now to FIGURE 4, an alternate system is shown which requiresno oil and which is adapted to process products in any of a variety ofcontainers as well as products which are unpac'kaged.

A tunnel 82 is provided with a vertical entrance section 83 and aninclined exit section 84, both sections having upper ends which passthrough an elevated platform 86. A horizontal microwave heating andholding tunnel section 87 extends between sections 83 and 84 at a levela small distance above the lower ends thereof, sections 83 and 87 beingconnected by a first inclined transition section 88 and sections 87 and'84 being connected by a second inclined transition section 89.

The horizontal tunnel section 87 is of enlarged cross section relativeto the other sections of the tunnel 82 with the upper portion 91 beingpartitioned from the lower portion 92 by a flat plate 93. Plate 93 andthe wall of the horizontal tunnel section 87 are formed of a goodelectrical and thermal conductor such as copper.

A waveguide 94 extends along the top wall of the initial half ofhorizontal tunnel section 87 and is coupled to a microwave source 96,the waveguide being adapted to inject microwave downwardly into theupper portion 91 of the tunnel section 87 in the manner previouslydescribed.

The product conveyer in this embodiment is a continuous belt 97 whichextends through tunnel 82 and between the upper ends thereof aboveplatform 86. Belt 97 is supported by a series of rotatable drums 98 ofwhich individual ones are situated at the top and bottom of entrance andexit sections 83 and 84 and at opposite ends of the horizontal section87. Additional parallel and adjacent drums 99 are associated with theuppermost drums 98 and rotary drive is applied to at least set thereofas indicated by arrow 101.

Belt 97 is formed with side portions 102 and spaced apart transversemembers 103 which extend between the side portions to form a continuousseries of pockets 104 along the inside of the belt for receiving, inthis example, plastic cans 106 of the product to be treated. Toprovidefor the free circulation of tunnel water around the containers 106, thebelt 97 including side portions 102 and transverse members 103 is formedlargely of an open mesh or screen material which is preferably adielectric, glass fiber screen being a suitable material.

Containers 106 are continuously fed into the pockets 104 of belt 97above the tunnel entrance section 83, an inclined tray 107 beingadjacent the inner side of the belt to deliver the containers thereto.Containers 106 drop from the belt 97 after it passes around the drum 98at the top of tunnel exit section 84 and are received on a secondinclined tray 108 disposed thereunder.

To provide hydrostatic pressure, tunnel entrance and exit sections 83and 84, transition sections 88 and 89, and the lower portion 92 ofhorizontal section 87 are filled with water 109. Water 109 is kept outof the upper portion 91 of tunnel section '87 by the air or gasentrapped therein, the water level being controllable by admitting highpressure gas thereto from a source 111, or releasing gas therefrom, bymeans of a valve 112.

A pump 113 withdraws water 109 from the top of entrance tunnel 83,through a filter 114, and discharges into the top of exit section 84through a water temperature regulating unit 116. Thus a continuous flowthrough tunnel 82 is maintained, the flow being counter to the directionof movement of the product therethrough.

As in the first embodiment of the invention, it is desirable that therebe a uniform high temperature within the horizontal tunnel section 87and a temperature gradient which increases with depth in both entranceand exit sections 83 and 84. Once established, these conditions areautomatically maintained to some extent by the inherent heat transfercharacteristics of the system. Thus the relatively cool water deliveredto the top of exit section 84 is heated, as it flows downwardly therein,by the countermoving heated product. The water flow thus arrives athorizontal section 87 at a temperature approaching the operatingtemperature therein. As the water flow moves upwardly through entrancesection 83, it is cooled by heat transfer to the incoming producttherein.

Due to such factors as incomplete heat exchange between water andproduct, heat loss through the tunnel walls, the heat input frommicrowave source 96, etc. the above described automatic temperaturecontrol will not generally maintain precisely optimum conditions.Accordingly temperature regulator 116 is utilized to hold the watertemperature at the top end of exit section 84 at the desired value. Forsimilar reasons a supplementary water temperature control system 117 isprovided at the horizontal tunnel section 87. System 117 includes a pump118 which withdraws a portion of the water flow through section 87,through a filter 119, and passes the withdrawn flow through a secondtemperature regulator 121 from which the flow is returned to the tunnelsection 87. Thus the desired temperature may be maintained in section 87irrespective of deviations of the overall system from preferredconditions.

In this embodiment, the flow of high temperature water through the lowerportion 92 of tunnel section 87 acts to maintain the temperature of thethermally conducting walls of the tunnel section, and the atmospheretherein, at the desired high temperature so that no significant heatloss occurs from the surface of the product by radiation, convectionetc. In some modifications of the invention, the water flow may not passthrough the tunnel section 87. The flow may, for example, be through abypass or separate flows may be circulated through the two end sectionsof the tunnel. Under these circumstances, means such as electricalheating coils or the like may be employed to heat the walls of themicrowave heating chamber 91, or the walls of the chamber can beconstructed of lossy material which will cause heating by the microwavepower.

In operation, product containers 106 are fed into successive ones of thepockets 104 of belt 97 and are carried downwardly into entrance tunnelsection 83. The surface regions of the product are heated in passagethrough entrance section 83 and the product is subjected to anincreasing pressure. The containers 106 are then carried into the firsthalf of upper portion 91 of section 87 where the interior of the productis heated by microwave energy. The product then remains at the elevatedtemperature while being carried through the terminal half of section 87.

Upon being carried upwardly through exit section 84, the product iscooled while the ambient pressure progressively decreases. Thus theproduct has been rapidly and uniformly heated to a high temperature, ina high pressure environment, held thereat for a predetermined shortperiod of time, and subsequently cooled.

It will be apparent from the foregoing description that varying forms ofapparatus may be utilized to practice the invention. The basic steps forobtaining the advantages of microwave heating under high pressure and ona continuous process basis are that a water free region be maintainedbeneath a volume of liquid and that the products to be treated becontinuously carried downwardly through the liquid and subjected tomicrowave energy within the water free region. A downwardly increasingtemperature gradient is maintained in the volume of liquid and theperiod required for the passage of the treated product upwardly throughthe liquid is adjusted so that 10 the product is cooled at least belowthe boiling point of its constituents prior to emerging from the liquid.

Accordingly, while the invention has been described with respect tocertain exemplary embodiments, many variations are possible within thescope of the invention and it is not intended to limit the inventionexcept as defined in the following claims.

What is claimed is:

1. In a method for subjecting substances to high temperatures in a highpressure environment, the steps comprising:

(a) establishing a downwardly increasing temperature gradient within avolume of liquid,

(b) maintaining a water free region below the surface of said volume ofliquid at a high temperature zone thereof,

(c) passing said substances downwardly into said liquid and into saidwater free high temperature region therein,

(d) exposing said substances to microwave energy while in said waterfree region, and

(e) gradually withdrawing said substances upwardly from said water freeregion through said liquid whereby said substances are cooled as thepressure therearound decreases.

2. In a method for subjecting substances to high temperatures in a highpressure environment as described in claim 1, the further stepcomprising:

(f) continuously moving successive increments of said substancesdownwardly through said liquid and through said water free region andthen upwardly through said liquid.

3. In a method for heat treating substances under high pressureconditions, the steps comprising:

(a) maintaining a gas filled region below the surface of a volume ofwater,

(b) electrically shielding said gas filled region from said volume ofwater,

(c) transporting said substances downwardly through said volume of waterto said gas filled region,

(d) .1njecting microwave energy into said substances Wlthin said gasfilled region, and

(e) gradually withdrawing said substances from said gas filled regionupwardly through said volume of water.

4. In a continuous process for the high temperature short time heattreatment of food products and the like, the steps comprising:

(a) maintaining a downwardly increasing temperature gradrent within avolume of liquid,

(b) maintaining a water free region within said volume of liquid at ahigh temperature and high pressure zone therein,

(0) continuously conveying successive increments of said productsdownwardly through said volume of l1qu1d and through said water freeregion thereof,

(d? exposing said products to microwave energy with- 1n said water freeregion to provide a uniform high temperature throughout said products,

(e) maintaining said uniform high temperature of said products for apredetermined period of time, and

(f) subsequently withdrawing said products upwardly through said volumeof liquid.

5. In a method for heat treating substances under high pressureconditions, the steps comprising:

(a) passing said substances through a volume of liquid,

(b) excluding water from the vicinity of said substances to create awater-free region about said substances while said volume of liquid isdisposed so as to eltect a hydrostatic pressure in the water-freeregion, and

(c) injecting microwave energy into said substances while in thewater-free region.

6. In a method for heating substances in a high pressure environment,the steps comprising:

(a) immersing said substances in a water-free liquid so as to efiect ahydrostatic pressure upon said substances,

(b) injecting microwave energy into said substances while saidhydrostatic pressure is asserted upon said substances,

(0) gradually withdrawing said substances from said liquid and 1 (d)cooling said substances in the course of Withdrawal from said liquid.

References Cited UNITED STATES PATENTS RICHARD M. WOOD, PrimaryExaminer. 0

L. H. BENDER, Assistant Examiner.

